1. Field of the Invention
The present invention relates to an image forming method, and more particularly to a method of forming monochromatic or dichromatic copy images easily with use of a developer composed of a highly resistive magnetic carrier and a nonmagnetic insulating toner which are triboelectrically chargeable.
2. Description of the Prior Art
In recent years, various copying machines have been proposed for making composite copies. With such machines, the usual step of exposure is followed by an exposure with use of a laser or an array of light-emitting diodes to form a composite latent image, which is developed and transferred. These machines are very useful for processing information. As disclosed specifically also in U.S. Pat. No. 4,346,982, the composite latent image thus formed has at least three different potential levels, so that the image area of a potential higher than the intermediate potential must be developed by the normal process, while the image area of lower potential needs to be subjected to reverse development. Accordingly when a two-component developer is used for development, there is the need to conduct normal development and reversal development separately. This requires two developing units, which render the machine large-sized inevitably. Further for the purpose of edition and discrimination, composite copying often involves the necessity of using different colors for developing the image obtained by primary exposure and the image additionally formed by secondary exposure. This gives rise to the need for two developing units, invariably making the copying machine large-sized.
In addition to the proposals for composite copying, various processes are also proposed for forming a dichromatic copy image from a single original image. For example, Published Unexamined Japanese Patent Application No. SHO 55-117155 discloses a process which employs a photosensitive member comprising a first photoconductive layer laminated to a conductive substrate and photosensitive to a first color (e.g. red), an intermediate layer over the layer, and a second photoconductive layer over the intermediate layer and photosensitive to a second color (e.g. black). Latent electrostatic images corresponding to copy images of first color and second color are formed in opposite relation in polarity by specified steps and then developed to visible images with two kinds of toners charged to a polarity opposite to each other and having different colors. Another process for forming dichromatic copy images is also proposed which employs a photosensitive member comprising a photoconductive layer laminated to a conductive substrate and having photosensitivity to both polarities as well as to two colors and in which latent electrostatic images corresponding to copy image areas of first color and second color are formed in opposite relation in polarity by specified steps and then developed with toners of two different colors. However, dichromatic copy image forming processes including these processes have the drawback of generally necessitating two developing units and therefore a copying machine of correspondingly increased size because two kinds of toners of different colors are used for development. Although a dichromatic developing method is proposed which uses a single developing unit, the method fails to give sharp dichromatic copies since fog or mingling of colors is liable to occur in copy images.